1. Field of the Invention
The invention concerns a device for roll-yaw attitude control of a satellite, usually a geostationary satellite, stabilized about three axes and having a continuous variable direction angular momentum.
In the present context a satellite is any artificial object in the solar system, being either in an orbit, circular or otherwise, about the Earth or any other planet or object in the solar system, or in a solar orbit, possibly a transfer orbit between two planets.
2. Description of the Prior Art
An orbiting satellite is known to be subject to disturbing torques with the following major causes:
asymmetrical effects of solar radiation pressure due to the angle of incidence of the pitch axis Y of the satellite relative to the Sun, the different reflectivity characteristics of the various parts of the satellite and the asymmetrical geometry of the satellite;
the action of the local (for example, terrestrial) magnetic field;
the effect of the local (for example, terrestrial) gravitational gradient (in the case of low orbits); and
It is essential to provide means for controlling the attitude of the satellite in its orbit. Known means for achieving this include active means using (in the case of a satellite whose attitude is stabilized about three axes):
a plurality of momentum wheels;
and/or thrusters;
and/or the solar radiation pressure (acting on appropriately oriented mobile surfaces coupled to the satellite);
and/or the local (for example, terrestrial) magnetic field (by generating magnetic dipole current loops on board the satellite).
The present invention is more particularly concerned with the currently widely used concept of a satellite stabilized on three axes and having a variable direction angular momentum (generated in practice by momentum wheels). A satellite of this kind usually has an angular momentum whose main component is along the pitch axis so that the pointing of the satellite about this axis can be controlled by varying the angular momentum, which must be desaturated from time to time by external action. The angular momentum, whose major component is along the pitch axis, has at least one other variable component (secondary component) whereby it can be oriented in at least one other direction (usually transverse to the pitch axis) to enable control about at least one other axis transverse to the pitch axis. This is usually the yaw axis to enable control of pointing in roll. The usual function of such secondary components of the angular momentum is to damp nutation (and possibly flexible modes) to which the satellite may be subject and to control in the short term, as mentioned above, the angle of orientation of the satellite perpendicular to this component (usually the roll angle when the angular momentum enables control about the yaw axis).
Numerous articles and patents have already proposed to use control of the direction of the angular momentum of a satellite to control its attitude.
The below-listed references are particularly noteworthy.
French Patent 2,525,359: Method and device of controlling the attitude of an artificial terrestrial satellite;
European Patent 0,071,445: Electronic roll/yaw satellite control;
U.S. Pat. No. 4,294,420: Attitude control systems for space vehicles;
French Patent 2,550,757: Satellite position control;
German Patent 2,537,577: Lagerregelung fur Satelliten;
French Patent 2,530,046: Geosynchronous satellite attitude control method and device;
French patent application 89-15732: Satellite roll and yaw attitude control method;
French patent application 89-17479: Attitude control system for using solar panels for a satellite stabilized about three axes;
unpublished French patent application 90-16149: Attitude control system for three axis stabilized satellite;
Lacombe, J. L.: Magnetotorquing for the attitude control of geostationary satellite--article published in: Proceedings of AOCS conference held in Noordwijk, the Netherlands, 3-6 October 1977, ESA SP-128, November 1977, pp. 103-110;
Tsuchiya, K., Inoue, M., Wakasuqi, N. and Yamaguchi, T.: Advanced reaction wheel controller for attitude control of spacecraft--article published in: Acta Astronautica, Vol. 9, no 12, 1982, pp. 697--702; and
Terasaki, R. M.: Dual reaction wheel control for spacecraft pointing--article published in: Proceedings of the symposium on attitude stabilization and control of dual spin spacecraft, Aerospace Corporation, El Segundo, California, August 1967.
When the satellite is in the mission normal mode (telecommunications or observation, for example, or scientific in nature), the objective of the attitude control system is to point the payload (antennas, optical instrument, radar, etc.) by maintaining the roll and yaw angles and drift thereof within assigned, possibly variable ranges.
To this end, appropriate control logic units simultaneously process sensor angular measurements and if necessary the speed of rotation of the wheels generating the angular momentum measured by tachometers and associated sensors in order to define set point angular momentum (or speeds) for these wheels and command torques for other actuators of the satellite.
In this type of attitude control, two control loops or logic units are typically used:
a fast loop whose objective is to reduce the roll and yaw angular speed and generally to provide short-term control of roll and/or yaw; attitude control can be effected by the wheels; this fast loop can be of any known type and does not form any part of the present invention; and
a slow loop adapted to compensate the effects on the satellite attitude of external disturbing torques, representing longer term control, by commanding torques produced by the other actuators.
Known fast loops include those described in the aforementioned articles by Tsuchiya et al. and Terasaki.
Various types of slow loops are known, each type being usually adapted to suit the actuators and the measuring device with which the satellite is provided. One example is described in the aforementioned article by Lacombe.
An object of the invention is to achieve a high level of attitude pointing performance (in respect of accuracy and speed in particular), at least as good as with known solutions, minimizing the mass of the components required for such attitude pointing, their cost and their electrical power consumption, thereby making them simpler and more reliable.
To this end, the invention teaches the addition to the short-term control of angular momentum direction long-term control to minimize the torques required of the actuators controlling satellite attitude in the longer term. If these actuators are of the magnetic type, for example, and s the variable (or secondary) component of the angular momentum is in a fixed direction, their number may be reduced from two to one (with equivalent pointing performance), the one magnetic actuator being disposed to generate torque in the same direction as the secondary component of the angular momentum. The magnetic coil has two functions: not only long-term control of satellite attitude in roll but also maintaining a value of the secondary component of the angular momentum such that the rotation of the satellite in its orbit provides for long term control of the yaw angle.
It will be remembered that orbital rotation of a satellite interchanges the roll and yaw axes.
For example, an angular momentum in yaw becomes after one-quarter orbit an angular momentum in roll (assuming no disturbance and no active control). With respect to one point of the orbit, it can be said that the presence of the angular momentum in yaw causes a variation of the angular momentum in roll which is physically equivalent to a torque in roll. The invention proposes to use this torque to control the attitude of the satellite.
For equivalent performance, the invention can therefore minimize the cost, the mass, and the electrical power consumption of the attitude control system.
As already mentioned, many articles and patent specifications h proposed to use control of the direction of the angular momentum of the satellite for short-term attitude control. To the knowledge of the inventor, the only disclosure which considers adding control by the slow loop to angular momentum direction control by the fast loop is the inventor's own French patent application 89-15732. However, in this case the object is to temporarily store in the angular momentum a difference in the control torque due to a shortcoming of the continuous actuator employed. Thus no author has yet proposed to control the direction of the angular momentum in order to deliberately reduce the amplitude of the torque required of the continuous actuators.
To the contrary, the invention goes against the conventional wisdom in this art which is to avoid mixing short-term and long-term control on the same actuator.